So I just bought a pre-paid Samsung Galaxy on5 that's Simple Mobile branded. I asked a Tmo rep beforehand whether or not that would work on Tmo post-paid plan and they said yes it would. But when I insert the Tmo sim card it says that the phone is network locked. (And i saw on another forum that someone was interchangeably using their Simple Mobile branded phone on simple mobile & tmo sims so i went ahead and bought the pre-paid phone.)

Does this mean that Simple Mobile branded phones won't work with Tmo network (even though they use the Tmo network) unless you get it unlocked from simple mobile? (That requires 12 months of pre-paid service).

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Sorry to hear about the confusion. Yes, since it's saying it's network locked, you will need to request the phone be unlocked in order to use it with T-Mobile. I would also suggest taking a look at our Bring your own device setup guide. It's got some important information for folks looking to use a phone from another carrier on our network.

Me too. Simple mobile say I need 12 month of service with them then they would provide unlock code but in the box doesn't say 12 month of pre-paid service. FYI Don't every buy Simple mobile pre-paid phone.